I have bought several vintage Martins on EBay. I have purchased high end instruments and cameras with no trouble ever. Buy from someone in the USA, be sure they have 100% positive reviews and many transactions.
Agree with Hans.
Will add that EBay/PayPal currently has an extemely buyer-biased selling system. How so? Regardless of a seller stating "as is" or "no returns" on the listing, if the buyer wants to return the item for any reason--not as expected, doesn't sound right, even good old-fashioned buyers' remorse (I'm not kidding)--then the seller MUST take the item back and PayPal will automatically refund the full purchase price. The seller cannot just say, "Eh, what the heck, I'll take a bad feedback, I'm not taking it back" as PayPal will give the buyer the refund and the rogue seller will be Sh*t out of luck. So, yeah, the seller pretty much has to take it back.
Note: the buyer pays the return shipping, even if the thing is completely misrepresented by the seller, so there's the small "pinch", but, to be honest, the buyer paying return shipping is true of many online transactions these days. The two times, out of about 600 transactions as a buyer on EBay, that I had to ship something back, the seller did refund even my shipping (they actually have to manually go into PayPal and gift you the shipping money). Why? Because you, the buyer, will still be writing feedback for them, so most will not want bad feedback because they were poopy about merely refunded shipping (as you likely know already, the seller can no longer leave a fast-paying buyer bad feedback, only positive feedback). Both times I had to ship something back were ukuleles, one a Martin and the other a vintage a Gretsch, and both deliberately understated the described condition (a classic: "no cracks" when there are cracks, they're just not described or photographed) or left out some key defects (such as dings) by cunningly not photographing the ukes properly.
A note to sellers: the buyers have you by the short-hairs, so best to overstate defects and take photos that over-emphasize the issues, not minimize them. Also, avoid words like "perfect", "mint", "like new" or "no flaws" in the selling description as (a) you're likely wrong, and (b) it sets up unrealistic expectations in buyers; you are more likely to process a return, which no one--buyer, seller or EBay--wants, than if you'd been as open and visible as a highway billboard to begin with.
Hans is right, though, stick with those sellers with plenty of astonishing feedback. I secured my entire vintage collection through EBay. Best of luck.